Porosity ... Tubes, though they hold air very effectively, seep air. Air sneaks out through the bead occassionally too going over bumps.
10 lbs. is about avg. per month, better than.. if you frequently ride.
I've had bikes idle for many months and had only a little loss. Others required routine topping-off. In which case, tubes were changed.
There IS a point though where the air escapes at a reduced rate.
Cars are a good example; less pressure (especially when u.infl.) with plenty of volume. Bikes have Hi-Press, low volume. Golf Carts have neither.
If you routinely kept your bike tires too low, they would lose less.